Conventional metal hydride adsorption systems for producing a hydrogen-rich product from a hydrogen-containing feed gas consist of four adsorption beds and four cycle steps. The four cycle steps are: adsorption, rinsing, depressurization, and repressurization. In such a system, a feed gas enters the vessel currently on line for the adsorption step at a certain temperature and pressure dependent upon the front end operations necessary to reduce the "poison" concentrations in the feed gas. The adsorption step continues until the tail gas; i.e. feed effluent, composition is equal to the feed composition, indicating that the adsorption capacity of the bed has been achieved. The tail gas from the first vessel is subsequently used to repressurize the adsorbent bed in a different vessel which has just completed the depressurization step.
The adsorbent bed in the first vessel now consists of metal hydride onto which pure hydrogen has been adsorbed and also gas of equal composition to the feed gas located in the void space of the bed and vessel. The rinsing step is then carried out using product hydrogen to displace the feed gas in the voids. This reduces the amount of impurities that will be present during the subsequent depressurization step. Since the pressure of the rinse gas should be approximately that of the feed gas and the product gas is at some reduced pressure, rinse compression is necessary.
After the rinse step has been completed; i.e. rinse effluent concentration is equal to product concentration, the bed is depressurized to some optimum pressure, dependent upon both the mole fraction of hydrogen in the feed gas and the feed pressure. Product hydrogen is drawn off and the bed is subsequently repressurized using tail gases from another vessel on line. This entire four step process is then repeated.
Several variations of this basic type of metal hydride adsorption process have been disclosed. Reilly, et al. U.S. Pat. No. 3,793,435 discloses a process for separating hydrogen from other gaseous products such as O.sub.2, N.sub.2, CO, CO.sub.2, H.sub.2 O, and CH.sub.4. Separation is achieved by contacting the gas mixture with a distributed form of an alloy of a rare earth lanthinum and nickel in an active state, the metal forming a hydride on contact with the hydrogen. Hydrogen separation is effected by passing a gas mixture at an initial pressure of 175 psia through a tubular reactor containing an inert, high porosity packing to prevent agglomeration at room temperature, removing an effluent gas at a pressure of 116-133 psia and then desorbing by reducing the pressure to 25 psia.
Billings, U.S. Pat. No. 4,108,605 discloses a process for treating a mixture of hydrogen and impurity gases by passing the gas mixture to a hydride container where the hydrogen is absorbed by the hydride forming material. To facilitate absorption and release of the hydrogen and impurities from the hydride forming material, a cooling fluid pump and heat exchanger and a heating fluid pump and heat exchanger are provided.
Sandrock, et al., in U.S. Pat. Nos. 4,079,523; 4,096,639; and 4,096,641 disclose various hydridable alloys which are suited for recovering hydrogen from a gas stream.
Turillon, et al., in U.S. Pat. Nos. 4,135,621; 4,134,490; 4,134,491 and 4,133,426 disclose the use of specially extended particles of solid having low apparent density distributed within a mass of metal hydride as a system for storing hydrogen. Mixtures of hydridable metal and particulate solids, i.e. metal powders such as nickel powder, are added to the hydrogen storage vessel. Also disclosed are the use of dry diatomaceous earth, lining, and metal whiskers in the powder combined to provide a packing density of not greater than 30% of the theoretical density of the powder itself. Additionally, the U.S. Pat. No. 4,134,491 discloses the use of collapsible structures.
Sheridan, et al. U.S. Pat. No. 4,360,505 discloses an improved adiabatic process for separating hydrogen from mixed gas streams using hydridable materials as the absorbing medium. The improvement involves utilizing a composite comprising a thermal ballast in admixture with the hydride material to absorb the heat of the reaction and to aid in desorption. By virtue of the intimate contact of the ballast with the hydridable material, rapid cycle times plus good bed utilization are achieved.